The present invention relates to a device for correcting brightness roll-off at an end of a lamp, and is particularly directed to such a device which gradually corrects brightness roll-off at an end of a tubular lamp to produce a uniform illumination of an imaging field of a contact image sensor (CIS) type image camera.
A tubular fluorescent lamp is typically used to illuminate the imaging field of a CIS type image camera. A characteristic associated with a tubular fluorescent lamp is that the brightness output level at each end of the lamp rolls off gradually. This gradual roll-off in brightness output level at each end of the lamp results in a non-uniform illumination of the imaging field of the camera.
A known approach in attempts to produce a uniform illumination of the imaging field is to use a longer lamp so that only the middle flat portion of the lamp is used for illumination. A disadvantage of using this approach is that a larger lamp must be used. This results in greater power consumption and more heat dissipation. Another known approach in attempts to produce a uniform illumination of the imaging field is to place a reflective plate along the longitudinal extent of the lamp. The use of a reflective plate creates a relatively bulky package. Accordingly, this approach is undesirable when a more compact package is desired.